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Today's topic is how to use colons.
One of my favorite grammar books, titled* Punctuate It Right, has a wonderful name for the colon: the author calls it the mark of expectation or addition (1). That's because the colon signals that what comes next is directly related to the previous sentence.
Use Colons After Complete Sentences
The most important thing to remember about colons is that you only use them after statements that are complete sentences. Never use a colon after a sentence fragment.
For example, it's correct to say, "Grammar Girl has two favorite hobbies: watching clouds and seeing how long she can stand on one foot." That's correct because "Grammar Girl has two favorite hobbies" is a complete sentence all by itself.
Notice how the items after the colon expand on or clarify what came before the colon. I referred to my favorite hobbies before the colon and then specifically named them after the colon. A quick and dirty way to decide whether a colon is acceptable is to test whether you can replace it with the word namely. For example, you could say, "Grammar Girl has two favorite hobbies, namely, watching clouds and seeing how long she can stand on one foot." Most of the time, if you can replace a colon with the word namely, then the colon is the right choice.